Feature request: support `Date` objects as traits

OpenFeature supports [Datetime types](https://openfeature.dev/specification/types#datetime) for context attributes, but [Flagsmith traits do not](https://github.com/Flagsmith/flagsmith-js-client/blob/26ae6181b38a569b09db1af9f4210d1a8fe3fa98/types.d.ts#L3).

Because of this, the OpenFeature provider has to make a copy of all the traits and map any `Date` objects into strings, which we've added in https://github.com/open-feature/js-sdk-contrib/pull/1351. The OpenFeature provider could potentially have some smarter logic/caching to avoid this copying, but this is overkill for what should be a simple adapter on top of the Flagsmith SDK.

Ideally, the Flagsmith SDK should natively support `Date` objects, and only serialise them into strings when needed. This avoids having to make a copy of the entire evaluation context whenever it changes, and makes the OpenFeature provider implementation simpler and faster.